Cuttlefish are marine animals of the order Sepiida. They belong to the class Cephalopodan (which also includes squid, octopuses, and nautiluses). Despite their name, cuttlefish are not fish, but molluscs.
Cuttlefish have an internal shell (the cuttlebone), large W-shaped pupils, and eight arms and two tentacles furnished with denticulate suckers, with which they secure their prey. They generally range in size from 15 cm (5.9 in) to 25 cm, with the largest species, Sepia apama, reaching 50 cm in mantle length and over 10.5 kg in weight.
Cuttlefish eat small molluscs, crabs, shrimp, fish, octopuses, worms, and other cuttlefish. Their predators include dolphins, sharks, fish, seals, seabirds and other cuttlefish. Their life expectancy is about one to two years. Recent studies indicate that cuttlefish are among the most intelligent invertebrates. Cuttlefish also have one of the largest brain-to-body size ratios of all invertebrates. They are sometimes referred to as the chameleon of the sea because of their remarkable ability to rapidly alter their skin color at will. Cuttlefish change color and light polarisation to communicate to other cuttlefish and to camouflage themselves from predators.